The actual size of the ZIP detectors in the CDMS experient are 1 cm thick and 3 inches(7.62 cm) in diameter. In order to increase the interaction event number, an artificial huge size detector in the simulation has the folloing dimension: 100m in length, 70m in diameter. Neutron beam from the top incident vertically onto the detector. Around the detector is vaccum, and the temperature is set to 50 mK.
There naturely exists 5 diffirent Germanium isotopes, all of them are found in the simulation.
Name: germanium
Symbol: Ge
Atomic number: 32
Atomic weight: 72.64
| Isotope | Atomic mass (ma/u) | Natural abundance (atom %) |
|---|---|---|
| 70Ge | 69.9242497 (16) | 20.84 (87) |
| 72Ge | 71.9220789 (16) | 27.54 (34) |
| 73Ge | 72.9234626 (16) | 7.73 (5) |
| 74Ge | 73.9211774 (15) | 36.28 (73) |
| 76Ge | 75.9214016 (17) | 7.61 (38) |
Neutrons have no electrical charge and have nearly the same mass as a proton. A neutron is hundreds of times larger than an electron, but one quarter the size of an alpha particle. Because of its size and lack of charge, the neutron is fairly difficult to stop, and has a relatively high penetrating power.
There are five reactions that can occur when a neutron interacts with a nucleus[1,2]. In the first two, known as scattering reactions, a neutron emerges from the reaction. In the remaining reactions, known as absorption reactions, the neutron is absorbed into the nucleus and something different emerges.
| 1. Elastic Scattering -- (n, n) | 2. Inelastic Scattering -- (n, n γ) |
|
A neutron collides nucleus, transfers some energy to it, and bounces off in a different direction. Target nucleus gains the energy lost by the neutron, and then at an increased speed. |
A neutron strikes a nucleus and is temporarily absorbed, forming a compound nucleus. This will be in an excited state. It then de-excites by emitting another neutron of lower energy, together with a gamma photon, which takes the remaining energy. |
|
|
3. Transmutation -- (n, p), (n, α)
A nucleus may absorb a neutron forming a compound nucleus, which then de-energizes by emitting a charged particle, either a proton or an alpha particle. This produces a nucleus of a different element. Transmutation is the transformation of one element into another by a nuclear reaction.
| (n, p) | (n, α) |
 |
 |
4. Radiative Capture -- (n, γ)
This is the most common nuclear reaction. The compound nucleus formed emits only a gamma photon. In other words, the product nucleus is an isotope of the same element as the original nucleus. Its mass number increases by one.
5. Fission
While bombarding heavy nuclei with neutrons, a highly excited compund nucleus might be formed. It might split into two nuclei of medium mass. During this process energy is produced and more neutrons are released.
The following picutres are view from side. Two red lines on the left and right sides are the top and bottom of the detector. The trajectories of diffirent particles are labeled by color according to their charges. Neutral particles are green, negative red, and positive blue. Most of the green lines in the following plots are nuetrons and gama rays , red lines are electrons, and blue lines are protons or alpha particles(recoil nuclei have very short moving distance before they dessipate all their enery into the crystal, so their trajectories are hardly to see).
It is worth to mention that the neutrons produced by radioactivity in the rock have energyies of a few MeV, while the neutrons from muon interactions in the surrounding rock cavern have energies well above 50 MeV. So We are mainly interested in the discussion of 2MeV and 20 MeV neutrons.
| Incident neutron energy 20keV | Typical interactions: | ||||||||||||||
Show plots for:
|
|
| Incident neutron energy 200keV | Typical interactions: | ||||||||||||||||
Show plots for:
|
|
| Incident neutron energy 2MeV | Typical interactions: | Recoil energy histogram | ||||||||||||||||
Show plots for:
|
|
 |
| Incident neutron energy 20MeV | Typical interactions: | |||||||||||||||||||
Show plots for:
|
|
 |
| Incident neutron energy 200MeV | Typical ineractions: | ||||||||||||||||||
Show plots for:
|
|
| Incident neutron energy 2000MeV | Typical ineractions: | ||||||||||||||||
Show plots for:
|
|
General structure of neutron cross sections[3]:
1. Most neutron absorption occurs in low energy (< 1 eV) range. The absorption cross section in this range tends to be inversely proportional to neutron velocity (i.e., 1/v). Germanium provides extremely high thermal neutron absorption cross sections.
2. The energy range from about 1 eV to the first inelastic scattering threshold (which is isotope-dependent) is dominated by elastic scattering , except in the vicinity of resonances.
3. Above the energy threshold for the lowest level of inelastic scattering, this mechanism tends to dominate the slowing-down processes, especially for heavier nuclides. This is true even if the cross section value for inelastic scattering is lower than elastic, because of the higher fractional energy loss associated with inelastic scatter. The secondary neutrons that emerge from inelastic scattering reactions tend to be isotropic in the COM system due to the formation of a compound nucleus -- and the subsequent "forgetting" of the original direction of the incoming neutron.
4. For details about the neutron interaction Q-values and threshold of each isotope of Germanium, see appendix.
|
|
|
|
|